1. Field of the Invention
The present invention relates to integrated circuit manufacturing, and more particularly to a method of stitching segments defined by adjacent image patterns of a photolithographic system during the manufacture of a semiconductor device.
2. Description of Related Art
Photolithography is frequently used in semiconductor fabrication to selectively expose regions of a material on a semiconductor wafer or substrate. Typically, the wafer is cleaned and prebaked to drive off moisture and promote adhesion, an adhesion promoter is deposited on the wafer, a few milliliters of photoresist are deposited onto the spinning wafer to provide a uniform layer, the wafer is soft baked to drive off remaining solvents, the wafer is put into a photolithographic system and exposed to radiation of an appropriate wavelength that transfers a master pattern from a reticle, and then the photoresist is developed. Positive photoresist, in which the developer removes the irradiated regions, is usually used. The photoresist is further hard baked to improve its etch resistance, and then the wafer is subjected to an additive process (such as ion implantation) or a subtractive process (such as etching) using the photoresist as a mask. Thereafter, the photoresist is stripped.
Photolithographic systems typically use an optical radiation source and a lens in conjunction with a mask or reticle to selectively irradiate the photoresist. The radiation source projects radiation through the mask or reticle to the lens, and the lens focuses an image of the mask or reticle onto the wafer. A mask transfers a pattern onto the entire wafer (or another mask) in a single exposure step or scanned exposure, whereas a reticle transfers a pattern onto only a portion of the wafer.
The three major methods of optically transferring a pattern on a mask or reticle to a photoresist-coated wafer include contact printing, proximity printing, and projection printing. In contact printing, the mask is clamped against a photoresist-coated wafer. Although this optimizes image transfer and resolution, the contacting process results in mask defects. In proximity printing, the mask and photoresist are spaced by a small distance. Although this overcomes the defect problems associated with contact printing, it also requires extremely flat wafers and masks. In projection printing, lens elements or mirrors are used to focus the mask or reticle image on the photoresist, which is spaced from the mask or reticle by a large distance. Projection printing is usually used for photolithographic pattern transfer in semiconductor fabrication and many technologies have been developed, including projection scanners and step and repeat systems. Projection scanners use a reflective parabolic mirror to project the mask onto the wafer by scanning the wafer and the mask with a narrow arc of radiation. Step and repeat systems (steppers) project an image only onto a portion of the wafer. Multiple images of the reticle pattern are stepped and repeated over the entire wafer using multiple exposures. The reticle pattern is typically 1X to 10X the size of the image pattern on the wafer, with reduction provided by the lens. Non-reduction (1X) steppers offer a larger field, thereby allowing more than one pattern to be printed at each exposure. In this manner, a single reticle can be used to create a very large pattern containing a repeated image pattern.
In some instances, it is desirable to provide continuous lines that span several exposure fields. However, it has been found difficult to precisely match up the edge of a previously exposed image pattern with the edge of the image pattern adjacent to the previously exposed image pattern. FIGS. 1A-1D show a conventional approach for correcting misalignment between segments defined by adjacent image patterns. In FIG. 1A, segments 100 and 102 are laterally displaced in the x-direction and are decoupled from one another. In FIG. 1B, segments 110 and 112 are laterally displaced in the y-direction but remain coupled to one another by a thin region. In FIG. 1C, segments 120 and 122 are laterally displaced in the y-direction and are decoupled from one another, and in FIG. 1D segments 130 and 132 are laterally displaced in both the x- and y-directions and are decoupled from one another. For electrically conductive segments, such as polysilicon and metallization such as aluminum, that are intended to form a continuous circuit, a discontinuity between the segments constitutes an open circuit. Furthermore, when conductive segments are coupled by a significantly smaller region than the linewidths (FIG. 1B), the coupling may be inadequate since the line resistance may become too high, and metal segments (particularly aluminum) become more susceptible to decoupling due to electromigration. In FIGS. 1A-1D, the segments are stitched using a conventional approach by depositing metal contacts 104, 114, 124 and 134, respectively (shown as broken lines) over the ends of the segments. A disadvantage to this approach, however, is that additional processing steps are required to deposit and etch the metal contacts.
Accordingly, a need exists for an improved method of stitching together segments defined by adjacent image patterns projected onto a photoresist layer so that segments are adequately coupled to one another.